History of Wireless Communications
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1
1897:
Marconi invented wireless concept
1960’s & 1970’s:
Bell laboratories developed the
cellular concept
1970’s:
Development of highly reliable, miniature
solid state radio frequency hardware
Wireless communication era was born
Evolution of wireless users

1984
1994
1997
2000
-
25,000
16 million
50 million
Number of wireless users =
Number of wired users
2010 – 5 billion users worldwide
2
Commonly used wireless systems
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3
Garage door openers
Remote controllers for home entertainment
Cordless telephones
Hand-held walkie-talkies
Pagers
Cellular telephones
Components of wireless system
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4
Mobile – Describes a radio terminal attached
to a high speed mobile (e.g., A cellular phone
in a fast moving vehicle)
Portable – Describes a radio terminal that can
be hand-held and used by someone at
walking speed (e.g., cordless telephone)
Subscriber – Mobile user
Base stations – Link mobiles through a
backbone network
Types of wireless transmission systems
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5
Simplex – Communication possible only in one
direction, (e.g., paging systems)
Half Duplex – Two way communication, but uses
the same radio channel for both transmission and
reception. User can only transmit or receive
information
Full Duplex – Simultaneous two-way radio
transmission and reception between subscriber
and base station
Types of duplex systems
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6
Frequency Division Duplex (FDD) -Two
simultaneous but separate channels
Time Division Duplex (TDD)-Adjacent
timeslots on a single radio channel
TDD can multiplicate number of channels in
FDD
Cordless Telephone Systems
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Full duplex communication
Usable range ~ hundred meters
Public
Switched
Telephone
Network
(PSTN)
7
Fixed
Port
(Base
Station)
wireless
link
cordless
handset
Wide Area Paging System
Landline link
PSTN
Paging
control
center
Landline link
Satellite link
8
City 1:
Paging
terminal
City 2:
Paging
terminal
City N:
Paging
terminal
Message format in Paging systems
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9
Numeric messages
Alpha-numeric message
Voice message
News headlines
Stock quotes
Faxes
Cellular System
Mobiles(Users)
Base
stations
(towers)
Mobile Switching
Center (MSC)
Public Switched
Telephone Network
(PSTN)
MSC
10
PSTN
Base Station-Mobile Network
RVC
RCC
FVC
FCC
FVC - Forward Voice Channel
RVC - Reverse Voice Channel
FCC - Forward Control Channel
RCC - Reverse Control Channel
11
Functions of Cellular System
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12
Provides wireless connection between users
and Public Switched Telephone Network
(PSTN)
PSTN is the wired network that includes
coaxial, microwave, fiber optic, under-sea
cables, satellite.
PSTN is the telephone network that provided
sole service before we had cell phones
Cellular service merely extends the PSTN
service to the mobile location
How does cellular system work?
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13
Base stations provide wireless connectivity to
mobile users
Each base station limits control to its small
geographical area (2-3 sq. km) or cell.
High capacity of cellular network is achieved:
 by limiting the coverage to the cell
 By using concept of Frequency reuse
 However, frequencies are reused in cells
quite far away to minimize interference
Cellular system handoff and capacity
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14
Switching system, called handoff, enables
call to proceed uninterrupted when
user moves from one cell to another
Typical MSC handles 100,000 cellular users
and 5,000 simultaneous conversations
at a time
Steps in telephone call made to Mobile User
Incoming
Base Stations
Telephone
Call to
Mobile X
Step 1
Mobile
Switching
Center
PSTN
15
2, 6
5
4
3, 7
Mobile X
Cellular Process in call to Mobile User
Step 1 Incoming telephone call is received by MSC
Step 2 MSC dispatches request to all BSs
Step 3 BSs broadcast MIN over FCC
Step 4 Mobile acknowledges over RCC to local BS
Step 5 BS relays mobile reply to MSC
Step 6 MSC instructs local BS to initiate call
Step 7-1 BS signals mobile to use unused channel*
Step 7-2 Alert is transmitted over FVC to ring mobile*
* Simultaneous process
16
Steps in telephone call made from mobile user
Mobile
Switching
Center
PSTN
17
3
2
1
Telephone
Call Placed
by Mobile X
Cellular Process in call from mobile user
Step 1-1 Mobile dials MIN of called party to BS
Step 1-2 Mobile transmits SCM* to show signal power
Step 2 BS receives data and sends it to MSC
Step 3-1 MSC validates request
Step 3-2 MSC connects to called party via PSTN
Step 4 MSC validates unused channel to mobile
* Station class mark
18
Cellular Roaming
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Roaming allows subscribers to operate in
service areas other than the home area.
When a mobile enters a geographic area that is
different from its home service area, it is
registered as a roamer in the new service area.
Roaming is essential to maintain service for
users in areas other than their home network.
Roaming Protocol
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Periodically, the MSC issues a global command
over each FCC in the system, requesting
mobiles to report their MIN and
ESN over the RCC.
MSC registers users in two categories:
 Home users
 Visitor users or roamers
Roaming will be discussed further in the
Cellular Networks chapter.
Initial Frequency Spectrum for US Cellular
Channel Number
1  N  799
.03 N + 825
990  N  1023
.03 (N – 1023) + 825
1  N  799
.03 N + 870
990  N  1023
.03 (N – 1023) + 870
Channels 800-989 are unused.
21
Center Frequency (MHz)
Comparison of Mobile Stations
Required
Service Coverage infrarange
structure
TV
remote
control
Garage
door
opener
22
Com- Hardware Carrier Functionplexity
cost frequency ality
low
low
low
low
infra-red
transmitter
low
low
low
low
<100
MHz
transmitter
Paging
system
high
high
low
low
<1 GHz
receiver
Cordless
phone
low
low
moderate
low
<100
MHz
transceiver
Cellular
phone
high
high
high
moderate <1 GHz
transceiver
Comparison of Base Stations
Coverage Required
infraService range
structure
TV
remote
control
Garage
door
opener
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Com- Hardware Carrier Functionplexity
cost frequency ality
low
low
low
low
infra-red receiver
low
low
low
low
<100
MHz
receiver
Paging
system
high
high
high
high
<1 GHz
transmitter
Cordless
phone
low
low
low
moderate
<100
MHz
transceiver
Cellular
phone
high
high
high
high
<1 GHz
transceiver
The Cellular Concept
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The cellular concept was a major breakthrough
in solving the problem of spectral congestion
and user capacity.
Replaces single high power transmitter (large
cell) with many low power transmitters (small
cells), each providing coverage to only a small
area.
Frequency Reuse Concept
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Cells with the same letter, use
the same set of frequencies.
A cell cluster is outlined
in bold, and replicated over
the coverage area.
G
F
G
F
B
A
E
In this figure, the
cluster size, N, is equal to 7;
since there are 7 cells in the cluster.
B
A
E
C
D
C
D
G
F
B
A
E
C
D
Example of cellular structure
26
Courtesy: UK cellular plan for scanning telemetry
Why the Hexagonal Cell?
Factors required for cell
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Equal area
No overlap between cells
Possible choices for cell shape
S
28
S
S
A1
A2
A3
• For a given radius S, A3 provides maximum
coverage area.
• By using hexagon geometry, the fewest number of
cells covers a given geographic region.
• Actual cellular footprint is determined by the
contour of a given transmitting antenna.
Comparison of possible cell shapes
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For a given radius S, A3 provides maximum
coverage area.
Actual cellular footprint is determined by the
contour of a given transmitting antenna.
By using hexagon geometry, the fewest number
of cells covers a given geographic region.
Calculation of channel capacity
Channel capacity (C) of a particular area
is the defined as the number of users that
can be served by the cellular system.
 C = M*K*N
Where:
M = Number of clusters in the area
K = Number of duplex channels/cell
N = Number of cells/cluster
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Design of cluster size N
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Tessellation condition (no gaps between
cells)
N = i2 + ij + j2
where i and j are non-negative integers
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Example; i = 2, j = 1
N = 22 + 2(1) + 12 = 4 + 2 + 1 = 7
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31
Cluster sizes are 1, 4, 7, 12, 19…
Co-channel cells
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Co-channel cells are cells using the same
frequency channels
For each cell, there are six co-channel cells
in the first layer
Co-channel cells are designed to be as far
away as possible from each other to minimize
interference
Location of co-channel cells
N = 19; i = 3, j = 2
A
A
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A
A
A
33
A
A
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Move i cells
Rotate 120 degrees
counterclockwise
Move j cells
Reach co-channel
cell
Example problem
If a particular FDD cellular telephone system has
a total bandwidth of 33 MHz, and if the phone
system uses two 25 KHz simplex channels to
provide full duplex voice and control channels,
compute the number of channels per cell if
N = 4, 7, 12.
34
Solution
Total bandwidth = 33 MHz
Channel bandwidth = 25 KHz x 2 = 50 KHz
Total available channels = 33 MHz / 50 KHz = 660
N = 4 Channel per cell = 660 / 4 =
165 channels
N = 7 Channel per cell = 660 / 7 =
95 channels
N = 12 Channel per cell = 660 / 12 =
55 channels
35
Fixed Channel Assignments
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Each cell is allocated a pre-determined
set of voice channels.
If all the channels in that cell are occupied, the
call is blocked, and the subscriber does not
receive service.
Variation includes a borrowing strategy:
a cell is allowed to borrow channels from a
neighboring cell if all its own channels are
occupied. This is supervised by the MSC.
Dynamic Channel Assignments
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37
Voice channels are not allocated to
different cells permanently.
Each time a call request is made, the base
station requests a channel from the MSC.
The MSC then allocates a channel to the
requested call, based on frequency re-use
of candidate channel, cost factors.
Dynamic channel assignment is more
complex, but reduces likelihood of blocking.
Handoff Strategies
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38
Handoff is a key process in any cellular system
Handoff occurs when a mobile moves into a
different cell, and the MSC automatically transfers
the call to a new channel belonging to the new
base station
Handoffs must be performed successfully, as
infrequently as possible and not visible to users.
Handoff scenario: Improper handoff
Received signal level
Level at point A
Handoff threshold
Minimum acceptable
signal to maintain the call
Pn
Pm
Level at point B
(call is terminated)
Time
A
39
BS1
B
BS2
Pn– Pm = ∆
∆ too large- too many handoffs
∆ too small- call may be lost
Handoff scenario: Proper handoff
Received signal level
Level at point B
Level at which
handoff is made
A
40
BS1
Time
B
BS2
Handoff parameters and variations
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41
Each base station constantly monitors the signal
strength of all its reverse voice channels to
determine the relative location of each mobile
user with respect to the base station tower.
Mobile assisted hand-off (MAHO) Every mobile station measures the received
power from surrounding base stations and
reports these measurements to the serving base
station - Faster hand-off rate.
Inter-system handoff - One cellular system to a
different cellular system.
Interference and System Capacity
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Signal interference is a major limiting factor in
performance of cellular systems
Two main types of of interference:
 Co-channel interference
 Adjacent channel interference
Co-Channel Interference
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Cells that use the same set of frequencies are
called co-channel cells.
Interference between these cells is called cochannel interference.
Standard form of interference measurement
is SNR (Signal to noise ratio) of SIR (Signal
to interference ratio)
Co-Channel Interference
Signal to interference ratio (SIR) or S/ I for a
mobile receiver is given by:
S
I
S
= i
I i
i 1
0
S = signal power from base station
Ii = Interference power from ith co-channel cell
44
i0 = Number of interfering stations
Schematic of co-channel interference
A
A
D
D-R
A
45
A
D+R
R
D+R
A
D
D-R
A
A
Calculation of Antenna power
Base station antenna power at distance d
Po
d
Pr
 d 
Pr  P0 
d 

 0 
n
where n is path loss exponent
46
Calculation of SIR
S

I
R 
i
n
 D i 
i 1
n
0
io = total number of first layer interfering cells
47
Mobile at center of cell (Di = D)
S

R

n
I
D 

R  n

i
n
D  i0
1
i 1
n
0
D
Q 

R
48
S

I
3N
 3N 
n
i0
Mobile at cell boundary (maximum
interference)
S

R

I
2( D  R )  n  2( D  R )  n  2( D ) n
1

2(Q  1)  n  2(Q  1) n  2(Q )  n
n
49
Adjacent Channel Interference
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50
Interference resulting from signals with
neighboring frequency to the desired signal.
Interference occurs due to imperfect receiver
filters that allow spectrum leakage.
Can be minimized by:
 careful filtering and assignments
 keeping frequency separation between cell
channels as large as possible.
Trunking and Grade of Service
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51
Cellular radio system relies on trunking to
accommodate a large number of users with
limited spectrum.
Trunking - each user is allocated a channel
on a per-call basis; and upon termination of
the call, the channel is immediately returned
to the pool of available channels.
Initiated by Danish mathematician, Erlang.
Grade of Service (GOS)
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52
GOS is the ability of the user to access a trunked
system during the busiest hour of a week,
month or year (for example, 4 - 6 pm Friday is a
very busy air time)
Traffic intensity is defined as the measure of
channel usage for each user.
Cellular traffic is similar to traffic on the freeway,
with each lane corresponding to a group of
channels.
Measure of Traffic Intensity
Traffic
usage of of each user is:
A =  H
 - Average number of calls per sec.
H - duration of a call (sec.)
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53
Total traffic usage with U users:
A = U A Erlangs
Traffic load/channel:
Ac = U A / C
Blocked Calls Cleared System
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54
No queuing mechanism available for call
requests.
If no channels are available, the requesting
user is blocked without access and is free to try
again later.
Calculation of GOS for Blocked system

Assuming a finite number of available channels
C, and using queuing theory:
GOS = Probability (call is blocked)
=
55
A 
 
 C! 
k
C A

k  0 k!
c
Practical estimation of GOS
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56
AMPS cellular is designed for GOS = 0.02
This is called Erlang B formula (Appendix A) Figure 3.6 in book.
57
Blocked calls delayed system
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Queue is provided to hold calls until a channel
becomes available
Prob [Delay > 0 ] =

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58
A
c
k
C

1
A
A
c
A  C!(1  ) 
C k  0 k!
Prob [Delay > t sec] = Prob [Delay > 0] x e – (C-A) t / H
Blocked calls delayed system
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59
Average Delay D for all calls in a queued
system is given by:
D
H
CA
This is called Erlang C formula (Appendix A) Figure 3.7 of book
60
Example problem
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61
A hexagonal cell in a 4-cell system has
a radius of 1.387km, and a total of 60 channels
for the system.
If the load / user is 0.029 Erlangs,  = 1 call per
hour, compute the following for an Erlang C
system that has a 5% probability of a delayed
call.
a. How many users per square km
will the system support?
b. What is the Prob [ Delay > 10s ]?
Solution
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62
Cell radius = R = 1.387 km
Area covered per cell = 2.6 R2
= 2.6 (1.387)2 = 5 sq km
Number of cells per cluster = 4
Total number of channels per cell =
60 / 4 = 15 channels
... Solution
a. From Erlang C chart, GOS = 0.05, C = 15,

Traffic intensity A = 9.0 E

Number of users
= total traffic intensity / Traffic per user
= 9.0 / 0.029 = 310 users
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63
Number of users per sq. km =
310 / 5 = 62 users per sq. km.
... Solution
b. Prob [Delay > 10s] = Pr [Delay > 0 ] e –(C-A) t / H
= 0.05 x e –(15-9) 10 / H
H = A /  = 0.029 hr
= .029 x 60 x 60 seconds
= 104.4 seconds
64
Prob [Delay > 10s] = 0.05 e –(15-9) 10 / 104.4
= 0.0281
= 2.81%
Improving Capacity in Cellular Systems
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65
As demand for wireless services increases, the
number of channels assigned to a cell is not
enough to support the required number of
users.
Solution is to increase channels per unit
coverage area.
Increasing capacity by Cell Splitting
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66
Subdivides a congested cell into smaller cells,
each with its own base station.
Increases the capacity of a cellular system.
Increasing capacity by Sectoring
• Sectoring divides the cell into independent
segments.
• Each sector is controlled by its own individual
sectoral antenna.
67
Increasing capacity by Sectoring
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68
Achieves capacity improvement by essentially
rescaling the system.
Cell radius R is unchanged but the
co-channel ratio D / R is decreased.
Capacity improvement is achieved by reducing
the number of cells in a cluster, and this
increases frequency reuse.
Replacing a single omni-directional antenna at
base station with several directional antennas,
each radiating within a specified sector.
Zone Selector
Increasing capacity by Micro Cell Concept
Microwave or
fiber optic link
Tx/Rx
Base
station
Tx/Rx
The Micro
Cell Concept
Tx/Rx
69
(Adapted from
[Lee91b] © IEEE)
Micro Cell Concept
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70
Large control base station is replaced by
several lower powered transmitters on the
edge of the cell.
The mobile retains the same channel and
the base station simply switches the
channel to a different zone site and the
mobile moves from zone to zone.
Since a given channel is active only in a
particular zone in which mobile is
traveling, base station radiation is
localized and interference is reduced.
Advantages of Micro Cell Zone
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71
The channels are distributed in time and space
by all three zones are reused in co-channel
cells.
Advantage is that while the cell maintains a
particular coverage radius, co-channel
interference is reduced due to zone
transmitters on edge of the cell.
Practice Problem
The US AMPS system has 50 MHz of spectrum in
the 800 MHz range and provides 832 channels.
42 of those channels are control channels.
The forward channel frequency is exactly 45 MHz
greater than the reverse channel frequency.
72
… Practice Problem
a. Is the AMPS system simplex, half-duplex or
duplex? What is the bandwidth for each
channel, and how is it distributed between the
base station and the subscriber?
b. Assume a base station transmits control
information on channel 352 operating at
880.56 MHZ. What is the transmission
frequency of a subscriber unit transmitting on
channel 352?
73
... Practice Problem
c. The A side and B side cellular carriers evenly
split the AMPS channels. Find the number of
voice channels and number of control channels
for each carrier?
d. For an ideal hexagonal cellular layout which
has identical cell sites, what is the distance
between the centers of the two nearest cochannel cells:
 For 7 cell reuse?
 For 4 cell re-use?
74
Solution
(a.)AMPS system is duplex.
Total bandwidth = 50 MHz
Total number of channels = 832
Bandwidth/channel = 50 MHz / 832 = 60 KHz
60 KHz is split into two 30 KHz channels (forward
and reverse channels).
Forward channel is 45 MHz > reverse channel.
75
Solution
(b.) For Ffw = 880.560 MHz
Frev = Ffw – 45 MHz = 835.560 MHz
76
Solution
(c.) Total number of channels = 832 = N
Total number of control channels Ncon = 42
Total number of voice channels Nvo =
832 – 42 = 790
Number of voice channels for each carrier = 790 /
2 = 395 channels
Number of control channels for each carrier = 42 /
2 = 21 channels
77
Solution
(d.) N = 7
Q=D/R=
3N =
D = 4.58 R
N=4
Q = 12 = 3.46
 D = 3.46 R
78
21
= 4.58